The present invention relates to silicate glass compositions capable of transmitting light in the ultraviolet portion of the spectrum, and more particularly to alkali boroaluminosilicate glasses exhibiting both good ultraviolet transparency and a thermal expansion coefficient sufficiently high to enable the glass to be sealed to substrates of relatively high expansion such as alumina.
Glasses exhibiting good transmittance in the near ultraviolet portion of the spectrum (e.g. at 254 nm), are of interest for a number of applications, including optical components for spectral analyzers and lamp envelopes for UV-emitting lamps. A number of glasses designed specifically for the transmission of ultraviolet light (180-400 nm) are commercially available. However, due to a variety of factors, silicate glasses with exceptionally good ultraviolet transmission tend to have relatively low coefficients of thermal expansion and are generally not easily melted. For example, Corning Code 9741 glass, which was originally intended for use in germicidal and ozone lamps but which has more recently been employed to provide windows for EPROM chips, has a minimum of 80% transmittance through a 1 mm thickness at 254 nm, but has a thermal expansion coefficient of 38.times.10.sup.-7 /.degree.C.
The ultraviolet transmission characteristics of silicate glasses have been reviewed by G. H. Sigel, Jr., "Ultraviolet Spectra of Silicate Glasses: A Review of Some Experimental Evidence", J. Non-crystalline Solids, 13 (1973/74) 372-398. As noted by Sigel, pure fused silica exhibits excellent ultraviolet transmittance, having an absorption edge below 160 nm. However the addition to fused silica of modifying species, particularly including alkali metal oxides, shifts the absorption edge strongly toward the visible, with the extent of the shift depending upon the nature and concentration of modifying oxides included in the glass.
As also pointed out by Sigel, certain impurity irons exhibit strong ultraviolet absorption and must be excluded from the glass if high transparency in the ultraviolet is to be achieved. Transition metals such as iron and rare earth elements were identified as particularly effective UV absorbers.
Unfortunately, fused silica has a very low thermal expansion coefficient, rendering it unattractive as a candidate for sealing to materials such as metals or other ceramics. In addition, pure SiO.sub.2 is not easily melted and formed at temperatures economically attainable in manufacture.
Alternatives to the use of silica or silicate glasses include the use of oxides such as B.sub.2 O.sub.3 and P.sub.2 O.sub.5 or glasses containing large amounts of these oxides, or even the use of non-oxide glasses such as BeF.sub.2. The latter compound, however, is both hygroscopic and highly toxic, while glasses based on P.sub.2 O.sub.5 and/or B.sub.2 O.sub.3 tend to have poor chemical durability, especially against water attack. Phosphate and borate glasses also tend to have less favorable viscosity/liquidus characteristics, e.g., a lower viscosity at the glass liquidus temperature, and thus present melting and forming problems.
Published Japanese patent application JA No. 62-027346 describes a family of UV-transparent phosphate glasses for use as small window elements in erasable-programmable read-only memory (EPROM) microelectronic circuit devices. However, due to the substantial P.sub.2 O.sub.5 -content of these glasses (35-70% by weight), they would be expected to be less durable and harder to form than more conventional silicate glasses. Therefore, there remains a need for alternative silicate glass compositions exhibiting good ultraviolet transmittance, but with a higher thermal expansion coefficient and better melting and forming characteristics.
Alkali boroaluminosilicate glasses of the type presently sold for applications requiring high ultraviolet transmittance are described, for example, in U.S. Pat. No. 2,382,056. That patent discloses the effect of alumina (Al.sub.2 O.sub.3) additions to selected alkali borosilicate glasses to achieve improved ultraviolet transparency. The disclosed glasses typically contain 50-70% SiO.sub.2, 1-5% total of alkali metal oxides, 20-40% B.sub.2 O.sub.3 and 4-10% Al.sub.2 O.sub.3. They also contain substantial fluorine as an aid to melting. A preferred glass provided in accordance with the patent exhibits a transmittance in 1 mm thickness of about 5-10% at 185 nm and about 80% at 254 nm.
As suggested in the prior art, ultraviolet-transparent glasses fusible to alumina substrates would have specific utility in the manufacture of EPROM devices. These devices employ small windows of UV transparent glass sealed to the substrate, which glass should therefore match the thermal expansion of the substrate as closely as possible. Desirably, thermal expansion coefficients in the range of about 56-62.times.10.sup.-7 /.degree.C. would be preferred for this application, but existing UV-transparent silicate glasses are much lower in expansion.
Additional difficulties with borosilicate glasses of the commercial type, which contain high quantities of added fluorine, include fluorine volatilization from the glass during melting. Fluorine volatilization is a major environmental concern in manufacture, and also promotes the formation of cord in the glass, degrading homogeneity and giving rise to local variations in refractive index and physical properties which are particularly undesirable in small samples.
The addition of alkali metal oxides to silicate glasses is one expedient which has in the past been resorted to in order to soften the glass and increase the thermal expansion coefficient thereof. However this alone is not a viable option to modify known ultraviolet-transmitting glasses since, as noted above, alkali metal additions tend to move the absorption edge of borosilicate glasses toward the visible region of the spectrum.
Alkali boroaluminosilicate glasses exhibiting higher expansion coefficients and good optical quality have been developed for other applications not requiring good UV transmittance. For example, U.S. Pat. No. 4,130,437 discloses a family of alkali boroaluminosilicate glasses comprising lead oxide and silver halides which have good glass forming characteristics and can be melted to optical quality. However, these glasses are photochromic and have been employed mainly for ophthalmic and related applications; they do not exhibit good UV transparency.
Accordingly, it is a principal object of the present invention to provide novel glass compositions providing glasses with high ultraviolet transmittance and higher expansion coefficients than prior art UV-transmitting glasses, making them compatible with higher expansion substrates such as alumina.
It is a further object of the invention to provide glass compositions for UV-transmitting glasses which exhibit good optical quality and are substantially free of cord and other melting defects.
It is a further object of the invention to provide UV-transmitting glasses which exhibit good melting and forming characteristics so that they can be readily formed into high quality glass products such as cane.
Other objects and advantages of the invention will become apparent from the following description thereof.